Anodized aluminum is aluminum on which an aluminum oxide layer is grown by an electrolytic process in an acid electrolyte solution. The term “anodized” derives from the fact that aluminum on which the oxide is grown is the anode in the electrolytic process. This anodically grown aluminum oxide layer augments a naturally occurring layer, about 2 nanometers (nm) thick, which forms on aluminum when the aluminum is exposed to air. An anodically grown oxide layer can be between about 0.5 micrometers (μm) and 100 micrometers thick. Acid solutions used for anodizing include chromic acid, sulfuric acid, and certain organic acids.
Anodic aluminum oxide layers grown in acid solutions are porous. The porosity is required for the electrolytic process in order to allow the electrolyte solution to contact aluminum metal. After the electrolytic growth, the oxide layer is sealed by a corrosion inhibitor which may include a dye for providing colored anodized articles.
Recently developed laser marking methods of anodized aluminum are described in a paper, “Grayscale Marking of Anodized Aluminium Plate by Using Picosecond Laser and Galvanometer Scanner”, by M. Gedvilas et al, Journal of Laser Micro/Nanoengineering, Vol. 9 (3), pages 267-270 (2014). These methods employ a commercially available pulsed laser delivering a beam of pulses having a duration of about 10 picoseconds (ps). The pulses have a wavelength of about 1040 nm, and can be delivered with variable power and pulse repetition frequency (PRF). The beam of pulses is scanned over an anodized aluminum workpiece being marked in a manner such that there is some spatial overlap of pulses on the workpiece. Grayscale marking is achieved by adjusting the pulse power, PRF, and the degree of spatial overlap. Laser radiation fluence in a pulse incident on the workpiece is kept below an ablation threshold for the aluminum.
Applicants have attempted to reproduce results in the Gedvilas et al paper with limited success. Problems were encountered in particular with dyed anodized aluminum samples and with samples of different origin, presumably anodized by different processes. These problems included damage or delamination of the oxide layer during the marking process, or delamination and corrosion occurring during testing of marked samples in a salt-fog test. There is a need for a laser marking process which can be used with anodized samples of different color or different origin without damaging or adversely affecting the anodic oxide layer.